Build downloads two different external python binaries

[hzeller]

The build downloads two different external binary versions of Python

• external/python39_x86_64-unknown-linux-gnu/bin/python3.9
• external/rules_hdl_cpython/install/bin/python3.8

Maybe not too much of an issue in reality but probably something to be aware of. Would of course be cool if the different parts could settle one version.

The escape hatch to use the system Python does now work anymore --repo_env=PYTHON_BIN_PATH, so probably worthwhile removing that documentation.

[cdleary]

Is this because we need to update rules_hdl to be consistent with the XLS repo? cc @mithro who had been looking at that